1. Field of the Invention
The present invention relates to a laser machining apparatus for machining a workpiece by a laser beam, and in particular to a laser machining apparatus equipped with a high power laser for performing laser machining by frequently changing and precisely controlling a laser output.
2. Description of Related Art
It has been carried out to utilize a successively measured value of a laser output of a laser oscillator provided in a laser machining apparatus for stabilizing laser machining by the laser machining apparatus. In general, a part of the laser beam transmitted through a rear mirror arranged opposite to an output mirror of the laser oscillator is inputted into a power sensor for measurement of the laser output. In a case of a laser oscillator having a reflecting mirror for reflecting the laser beam emitted from a laser resonator, there can be adopted an arrangement for taking out the part of the laser beam by transmission through the reflecting mirror for the measurement.
Thus, the laser output power is measured on the real-time basis in the laser machining. The laser output measured on the real-time basis is utilized in the following applications.
(1) Utilization of the measured value as data representing a present value of the laser output in a feedback control of the laser output for stabilizing the laser output.
(2) Detection of an abnormal status of laser machining by an prominent increase of a laser beam reflected on a workpiece made of material having high reflectance with respect to the laser beam and returned to the laser oscillator.
(3) Diagnose of malfunction of the laser oscillator by abnormal decrease of the laser output.
There is an nonnegligible delay of response in the power sensor for measuring the laser output. Particularly for measurement of high-power laser, it is generally adopted a type of a power sensor in which the laser beam is converted into heat and heat flow rate is measured. The time required for response to a change of the laser output is considerably late in comparison with the time from issuance of a laser output command to an actual output of the laser beam.
In an ordinary laser machining apparatus, time required for an actual output of the laser beam to reach 95% of the laser output command from an issuance of the laser output command is approximately 0.0002 sec, whereas the time required for the measured value to reach 95% of the laser output command is approximately 3 sec. Thus, it takes a long time for the measured value of the power sensor to reflect the actual output, whereas a time period from issuance of the output command to the actual output of the laser beam is instantaneous, no to be satisfactory for the foregoing applications (1)-(3).
In order to explain the above circumstance using an example, FIG. 1 shows an example of a system configuration of a conventional laser machining apparatus in which the above applications (1)-(3) can be performed. As shown in FIG. 1, a laser oscillator 5 for outputting a laser beam LB to be irradiated on a workpiece W through a mirror 7 and a condenser lens 8 may comprise a YAG laser which is pumped by a pumping source 4 such as a pumping lamp and a pumping LED to which electric power is supplied from a laser pumping power supply 3.
A laser output command for controlling the laser output is issued from a laser output commanding section 1 to the laser pumping power supply 3 through a feedback control section 2 to supply an electric power to the laser pumping power supply 3. The laser output command is designated by a machining program or a manual operation on a control panel (not shown).
A laser power sensor 6 provided outside of a rear mirror 5a of the laser oscillator 5 regularly monitors the laser output (laser power). An output signal of the laser power sensor 6 is amplified by an amplifier 7 and compared with the laser output command, and a difference therebetween is inputted into the feedback control section 3. The laser power sensor 6 and the amplifier 7 constitutes laser output measuring means.
As a result of the comparison, if the measured value of the laser output is lower than the laser output command value, a signal indicating a positive difference is outputted to the feedback control section 2, and if the measured value is greater than the laser output command value, a signal indicating a negative difference is outputted to the feedback control section 2. The feedback control section 2 compensates the laser output command in accordance with the signal indicating the difference by a feedback control. The feedback control of the laser output to control the laser output to coincide with the command value is performed.
Further to the above feedback control, the output of the amplifier 7 indicating the measured value of the laser power is compared with the laser output command and a difference therebetween is inputted into an output abnormality detecting section 10. As a result of the comparison, if the measured value of the laser power is lower than the laser output command value, a signal indicating a positive difference is outputted to the laser power abnormality detecting section 10, and if the measured value is greater than the command value, a signal indicating a negative difference is outputted to the laser power abnormality detector 10.
The output power abnormality detector 10 determines whether or not the difference is within an allowable range and if the difference deviates from the allowable range, it is determined that an abnormality has occurred and a stop signal is issued to the laser output command means to stop the operation of the laser oscillator 5.
It is determined that an abnormality occurs in a case where a laser beam reflected on a workpiece and returned to the laser oscillator greatly increases by an abnormality of the laser machining on the workpiece which is made of a material having high reflectance of the laser beam (in the application of (2)), and in a case where the laser output abnormally decreases by a malfunction of the laser oscillator (in the application (3)).
The delay of response of the laser power sensor 6 will be described on a case where a laser output command as shown in FIG. 2a is issued from the laser output commanding section 1. In FIG. 2a, a pattern of a laser output command 101 is shown with an axis of abscissa representing time (sec.) and an axis of ordinate representing an output power (W). In this example, the laser output command 107 is designated by a machining program. The command value is 4 kW in an early section for 3.5 sec., and is 1 kW in a later section for 4 sec.
FIG. 2b shows a transition of the measured value 107 of the laser output power, a laser output command value 104 after subjected to the feedback control and a difference 110 between the laser output command and the measured value 107 in a case where the laser output command 101 is issued to the laser machining system with the feedback control of the laser output power, as shown in FIG. 1. In FIG. 2b, an axis of abscissa indicates time (sec.) and an axis of ordinate indicates output power (W).
As shown in FIG. 2b, a status where the measured value 107 is lower than the command value 101 continues for a considerable rise period (from 0 kW to 4 kW) immediately after an issuance of the laser output command 101 due to a delay of response of the power sensor 6. Accordingly, a signal indicating a luck of output power is issued to the feedback control section 2, so that the laser output command 104 increases to an upper limit of the laser oscillator 5 under the feedback control, as indicated by a reference numeral 51. Then, the measured value 107 approaches the command value 101 of 4 kW to terminate the above status and enters a state where the command value 101, the laser output command 104 subjected to the feedback control and the measured value 107 coincide with one another. Thus, there exists a time period in which an excessive power is outputted at the rise of the laser output command 101.
Then, immediately after the command value 101 is changed from 4 kW to 1 kW, the measured value 107 excesses the command value 101 for a while. Therefore, the signal indicating excessive output is inputted to the feedback control section to drop the compensated laser output command 104 by the feedback control to zero, as indicated by a reference numeral 52. As a result, the laser machining is stopped instantaneously.
The above status is terminated when the measured value 107 drops to the command value of 1 kW, to enter a state where the command value 101, the command value 104 after the feedback control, and the measured value 107 coincide with one another. There is a possibility of causing a period in which too small laser power, e.g. zero is outputted immediately after termination of the output command.
Further, in the case of abnormal increase of the reflected light from a workpiece to the laser oscillator by an abnormality of the laser machining, and abnormal decrease of the output power by a malfunction of the laser oscillator, it takes a long time for the signal indicating the abnormality to be inputted into the output abnormality detecting section 10, to fail in taking a rapid response such as stopping of the operation.
As described, an effective operation of the control system can not be performed until the measured value by the power sensor 6 reaches the actual laser output. There is a proposal of an arrangement with respect to the detection of abnormality of the laser machining in JP 2001-287059A. In this proposal, it is attempted to detect an abnormality of laser machining by comparing an output of a simulator of the power sensor based on the laser output command value with the measured value of the power sensor to compensate the response time of the power sensor to enable the abnormality detection of the output power immediately after the rise and fall of the laser output command.
However, in this arrangement, precision of the abnormality detection is not sufficiently high due to an influence of the feedback compensation of the laser output, as discussed. Thus, a first object of the present invention is to provide a laser machining apparatus capable of performing a precise detection of an abnormality of the laser machining in a system using the feedback control of the laser output.
Further, in the above arrangement, specifically in a rise of the laser output command, a feedback error increases to cause an overshoot, since the measured value of the power sensor is lower than the laser output command. If a gain of the feedback control is set to a lower value in order to avoid the increase of the feedback error, it would be difficult to make the actual laser output coincide with the laser output command value in a steady state.
In JP 8-168891A, it is proposed to solve the above problem by gradually connecting a feedback path in FIG. 55. However, it is a matter of course that it is preferable to effect the feedback control as soon as possible. Thus, it is a second embodiment of the present invention to provide a laser machining apparatus capable of effecting the feedback control immediately after the rise of the laser output command.
There is a difficulty in using the measured value of the power sensor for the feedback control in a case of a pulse oscillation output whereas the measured value of the laser output is substantially equal to the command value except periods immediately after the sudden change of the command value in a case of a continuous oscillation output. Specifically, if the laser output wave is in the form of an exact rectangle, the measured value can be obtained by multiplication of a pulse peak value and a pulse duty value, but it is usual that the actual waveform is not in the form of an exact rectangle. Further, in the case where a laser output in the form of a trapezoid or a triangle is adopted for advantage of machining performance, an averaged of the laser output measured by the power sensor can not obtained by a simple theoretical equation.
In respect to the above, it is proposed that a gap between the actual output power and the command value of the laser output in the form of a pulse is compensated by a feedforward control using a characteristic table. According to this proposal, the feedback control can be utilized in the pulse oscillation output for stabilizing the laser output.
However, the feedforward control is effective in a case where the pulse waveform has a substantially rectangular form, but in the case of a waveform significantly different from the rectangular form, it is impossible to perform a desired machining since the feedforward value is made excessively large. It is therefore a third object of the present invention to perform the applications in the case of the pulse oscillation output equivalent to those in the case of the continuous oscillation output by effectively using a characteristic table without need of the feedforward control.
Further, in the case of the laser machining such as laser piercing, laser marking and laser welding in which a considerable amount of the outputted laser beam returns to the laser oscillator, the laser output may drop due to the feedback control to fail in continuously perform the laser machining. It is therefore a fourth object of the present invention to precisely perform the feedback control and the output abnormality detection in such case.
The present invention provides a laser machining apparatus capable of shortening a response time of the feedback control and improving lowering of precision of abnormality detection of laser machining by influence of the feedback control by simulation of the measurement by the laser power sensor for measuring an output power of a laser oscillator, and comparison of the simulated value and the measured value of the laser output.
The following system configurations of a laser machining apparatus for machining a workpiece by a laser beam outputted from a laser oscillator in accordance with a laser output command are provided according to the present invention.
According to a first aspect of the present invention, the laser machining apparatus comprises: laser output commanding means for issuing the laser output command; laser power measuring means for measuring an output power of the laser beam and outputting a measured value of the output power including an error due to a response characteristic thereof; measurement simulating means for estimating the measured value outputted from the laser power measuring means based on the laser output command issued from the laser output commanding means and the response characteristic of the laser power measuring means, and outputting a simulated value of the laser power measurement; and feedback control means for feedback compensating the laser output command in accordance with a difference between the measured value outputted from the laser power measuring means and the simulated value outputted from the measurement simulating means.
In this system configuration, laser output abnormality detecting means may be provided for detecting an abnormality of the laser output based on a difference between the simulated value outputted from the measurement simulating means and the measured value outputted from the laser power measuring means.
The measurement simulating means may comprise measurement input estimating means for estimating a laser power representative value representing an averaged laser power inputted into the laser power measuring means based on the laser output command issued from the laser output commanding means, and estimates the measured value outputted from the laser power measuring means using the laser power representative value estimated by the measurement input estimating means.
The measurement input estimating means may store a data table containing data of the averaged laser power with parameters of a pulse waveform or a pulse peak value, a pulse duty and a pulse frequency, and obtain the laser power representative value using the data in the data table by interpolation or extrapolation.
Alternatively, the measurement input estimating means may store an approximation equation for expressing the averaged laser power by a mathematical function of a pulse waveform or a pulse peak value, a pulse duty and a pulse frequency, and obtains the laser power representative value using the approximation equation.
The measurement input estimating means stores a plurality of data tables or approximation equations for the measurement input estimation and selectively uses one of the data tables or one of the approximation equations in accordance with a machining condition.
According to a second aspect of the present invention, the laser machining apparatus comprises: laser output commanding means for issuing the laser output command; laser power measuring means for measuring an output power of the laser beam and outputting a measured value including an error due to a response characteristic thereof; feedback control means for feedback compensating the laser output command issued from the laser output commanding means in accordance with a difference between the measured value outputted from the laser power measuring means and the laser output command; and measurement simulating means for estimating the measured value outputted from the laser power measuring means based on the compensated laser output command by the feedback control means and the response characteristic of the laser power measuring means, and outputting a simulated value of the laser power measurement; and laser output abnormality detecting means for detecting an abnormality of the laser output based on a difference between the simulated value outputted from the measurement simulating means and the measured value outputted from the laser power measuring means.
In this system configuration, the measurement simulating means may comprises measurement input estimating means for estimating a laser power representative value representing an averaged laser power inputted into the laser power measuring means based on the compensated laser output command by the feedback control means, and estimates the measured value outputted from the laser power measuring means using the laser power representative value estimated by the measurement input estimating means.
According to a third aspect of the present invention, the laser machining apparatus comprises: laser output commanding means for commanding the laser output; laser power measuring means for measuring an output power of the laser beam and outputting a measured value including an error due to a response characteristic thereof; first measurement-simulating means for estimating the measured value outputted from the laser power measuring means based on the laser output command issued from the laser output commanding means and the response characteristic of the laser power measuring means, and outputting a first simulated value of the laser power measurement; feedback control means for feedback compensating the laser output command issued from the laser output commanding means in accordance with a difference between the measured value outputted from the laser power measuring means and the first simulated value outputted from the first-measurement simulating means; second measurement-simulating means for estimating the measured value outputted from the laser power measuring means based on the compensated laser output by the feedback control means and the response characteristic of the laser power measuring means, and outputting a second simulated value of the laser power measurement; and laser output abnormality detecting means for detecting an abnormality of the laser output based on a difference between the second simulated value outputted from the second measurement-simulating means and the measured value outputted from the laser power measuring means.
In this system configuration, the first measurement-simulating means may comprise a first measurement-input estimating means for estimating a laser power representative value representing an averaged laser power inputted into the laser power measuring means based on the laser output command issued from the laser output commanding means, and estimates the measured value outputted from the laser power measuring means using the laser power representative value estimated by the first-measurement input estimating means. The second-measurement simulating means may comprise a second measurement-input estimating means for estimating a laser power representative value representing an averaged laser power inputted into the laser power measuring means based on the compensated laser output command by the feedback control means, and estimates the measured value outputted from the laser power measuring means using the laser power representative value estimated by the second measurement-input estimating means.
In the foregoing system configurations, the laser machining apparatus may further comprises laser output command correcting means for correcting the laser output command issued from the laser output commanding means in accordance with operation data of the laser machining apparatus and/or an external signal.
The measured value outputted from the laser power measuring means may be substituted for an initial value of the simulated value by the measurement simulating means each time when the laser output commanding means changes the laser output command in accordance with a change of the machining condition.